Dynamical versus statistical production of Intermediate Mass Fragments at Fermi Energies

The emission probability of Intermediate Mass Fragments (IMFs) in non-central reactions has been investigated in collisions of heavy ^124Xe projectiles with the two different medium-mass targets of ^64Ni and ^64Zn at the laboratory energy of 35 A MeV. The two colliding systems differ only for the target atomic number Z and, consequently, for the isospin N / Z ratio. The probability of IMFs emission from the projectile-like fragment has been measured, showing an enhancement of the IMFs emission for the neutron rich ^64Ni target. Most of the observed projectile break-up yield is associated with the production of only one IMF, that is, a quasi-binary splitting of projectile in two fragments in a broad range of charge asymmetry. For the events with one IMF, the relative contributions of the dynamical and statistical emissions have been evaluated. We find an enhancement of dynamical break-up probability for the neutron rich target with respect to the neutron poor one. The analysis suggests influence of the target isospin in inducing the dynamical break-up of projectile-like fragments. The new data have been also compared with previous published results of ^112,124Sn + ^58,64Ni systems, in order to disentangle between isospin effects against system-size effects on the emission probability. The comparisons between previous and new data suggest that the dynamical break-up is determined by the N / Z content of both projectile and target; for the cases here investigated, the influence of the system size on the dynamical emission probability can be excluded.